Pathology—general—special. Morbid anatomy. Pathological chemistry;
Disease. Health. Death—Somatic—partial—necrosis. Syncope. Apuœa.
Asphyxia. Coma. Death from old age.
The principles and practice of Veterinary Medicine should embrace all
that is known of the causes, nature, symptoms, prevention and cure of
disease in domestic animals. Incidentally it includes diagnosis and
prognosis.
=Pathology= is the science which tells of the causes, and nature of
disease, and the functional and structural changes by which it is
characterized. In modern usage the term pathology is understood to refer
to the intimate nature of disease, but this necessarily involves an
enquiry into its sources and the predispositions to its occurrence; its
phenomena whether in changes of function or structure; and its results
in the form of perverted function, structural changes, degenerations,
dependent disorder, etc. The field of pathology is further divided into
general pathology and special pathology.
=General Pathology= treats of disease processes in their generic form,
and as they appear in many different diseases. Thus inflammation and
fever are the prominent phenomena in a great many different diseases
which differ in their seats, their causes, manifestations and results.
Inflammation and fever are therefore subjects of general pathology.
Similarly all forms of degeneration—fatty, fibrous, calcic, amyloid,
etc., are disease processes found in many different organs and under
very varied conditions and they are accordingly included in general
pathology. Hypertrophy and atrophy are also possible in every organic
tissue irrespective of kind or seat, they belong therefore to this
particular field.
=Special Pathology= on the contrary is confined to a particular disease
and not only elucidates the causes, phenomena and results of such
disease, but seeks to do this in such a way as to differentiate this
malady from all others however closely related to it. Thus inflammation
of a bone is known under the general name of osteitis, this may be due
to a great variety of different causes, and each would have its own
special pathology. The osteitis of simple mechanical injury is
essentially different from the osteitis of rheumatism, of purulent
infection, of tuberculosis, of actinomycosis. So with the inflammations
of every other tissue. Each may suffer from a variety of inflammations,
springing from different causes, attended with characteristically unlike
tissue changes and tending to different issues, and every one of these
forms has therefore its own special pathology.
=General Pathology= may be said to deal with typical disease processes
to a large extent irrespective of the individual disease in which they
may appear, while =Special Pathology= deals with the morbid phenomena
which distinguishes the individual malady from all other diseases
however closely allied to it.
=Pathological (Morbid) Anatomy= deals with structural changes, the
cause, the accompaniment or result of disease. These morbid changes are
microscopic or macroscopic. Both constitute morbid anatomy, but the
microscopic alterations come under the special name of morbid
(pathological) histology.
=Pathological Chemistry= is that branch of pathology which treats of
chemical changes produced by disease in the blood, lymph, tissues,
secretions and excretions. It demands a previous knowledge of the
condition of these tissues and fluids in health, in the particular genus
of animal and under the same dietary and environment. Physiological
Chemistry is therefore an essential prerequisite to pathological
chemistry, just as anatomy, physiology and histology are indispensable
to the appreciation of pathology and morbid anatomy.
=Disease= is an injurious deviation from the normal function or
structure. The morbid process resulting in disease is usually in the
nature of a modification of the normal or physiological condition, so
that it is often difficult to set the exact limits of health and
disease. What is a purely physiological process under given conditions,
would be distinctly pathological under others. The free kidney secretion
of cold weather and the profuse perspiration of a hot season are both
purely physiological and in the main balance each other. Each under its
special environment fulfills an essential work in eliminating from the
system toxic materials which would prove hurtful if retained, and thus
each is not only physiological but beneficial. If, however, they
occurred, not in this mutually compensatory manner, but simultaneously
in this profusion, they might well be dreaded as morbid conditions.
Again if either were to occur apart from its normal causative
environment, if for example the polyuria appeared in hot weather and the
perspiration in cold, the phenomenon might fairly be called
pathological. In any case if the excessive secretion induced a lowering
of the general tone of health the process would be essentially a morbid
one. In pronouncing therefore upon a morbid process one must take fully
into account the corresponding physiological process, the attendant
conditions, and whether the result is injurious or otherwise.
The same is true of structural changes. What under given conditions
would be essentially a morbid structure, might under other conditions be
a simple adaptation to an unwonted environment, and a means of
protection from injuries that would otherwise accrue. Excessive growth
of cuticular tissue in the epithelioma, wart or corn is injurious and
essentially pathological, while the callus on the camel’s knee or the
workman’s palm is purely protective and physiological. The local
development of a mass of fatty tissue in the average man or beast is a
disease, but the tendency to the uniform deposition of fat in the
connective tissue of the improved breeds of meat producing animals, is
the happy culmination of a long continued and skillful selection and
regimen, without which the live stock industry of today would be a
grievous failure. To constitute disease, therefore, modified function
must be permanent, and not simply a compensating increase, decrease, or
other change, and it must be in some way injurious to the animal
economy. Similarly to constitute disease modified structure must be
other than a simple protection or beneficial change, it must not be a
simple evolution in the nature of accommodation to the environment but
it must be a cause of injury to function or a distinct deformity.
=Health= may be said to be the harmonious exercise and mutual balance of
all the bodily functions, and any interference with such mutual exercise
or balance may be said to constitute disease. But as health passes into
disease by insensible gradations, there is of necessity an extensive
borderland which often cannot be allotted to one condition or the other,
but which must often be left a disputed territory.
Again certain animal constitutions are innately strong and robust, while
others are weak and feeble, yet the delicacy of the latter cannot be set
down as actual disease, and by maintaining a due balance between the
functions, a fair measure of health and even long life may be secured.
=Death= as the result of disease may be either _partial_ or _somatic_.
=Partial= or =local death= may be _molecular_ as in =ulceration=, or it
may affect an organ or part of an organ, as in =necrosis=, =sphacelus=,
or =sloughing=. =Somatic death= is a loss of vitality of the entire body
and is manifested by a complete cessation of the bodily functions,
including that of nutrition. Usually the arrest begins with one of the
great vital processes, in advance of the others, and thus in different
cases, we have _death beginning at the heart, at the lungs, and at the
brain_.
=Death from syncope or fainting, begins at the heart=, which loses its
irritability or contractility, or is seized with a tonic spasm. If there
has been lack of contractility, the heart is found after death in a
flabby, soft condition, and quite frequently filled with blood. If
heart-spasm, it is contracted, firm, and empty or nearly so. Syncope may
result from severe nervous shock (emotional), from the electric current,
from insolation, or from heart sedatives like chloroform, or nicotine.
It may, however be but the culmination of a gradually advancing
debility, from exhausting diseases, from fatty degeneration of the
cardiac muscles, or from starvation, or anæmia. Again the exhaustion
coming from profuse hæmorrhage, or from violent over-exertion is a cause
of fatal syncope.
In =death beginning at the lungs= (apnœa, asphyxia, or suffocation), the
blood failing to receive oxygen and to give up its carbon dioxide is
unable to maintain the various functions of the body and the arrest of
the other vital processes speedily follows. The arrest of the
respiratory process may occur from nervous shock, but more commonly it
results from choking, strangulation, drowning, or the action of
irrespirable gases. In diseases of the heart and lungs it is liable to
occur from the obstruction of the pulmonary circulation and from the
depression of the respiratory nervous centres. After death the lungs are
found gorged with dark red—almost black—blood, which likewise distends
the right heart and systemic veins, and all mucous and serous membranes
have a dark red, congested aspect. When breathing has been arrested by
mechanical violence there are, first, active contractions of the
respiratory muscles, but no loss of consciousness; then as the brain
becomes charged with venous blood, consciousness and volition are lost
and convulsive movements ensue. Later still there is no respiratory
effort nor convulsions, but the heart continues to beat for two or three
minutes longer.
=In death beginning at the brain= (Coma) the sensory functions fail
first, as evidenced by drowsiness, stupor, or complete insensibility,
while the movement of heart and lungs are still temporarily continued.
Pressure on the brain by a fractured bone or blood clot, or in cases of
violent congestion or the rapid growth of tumors, usually operates in
this way. It may also result from the direct action of certain poisons,
like opium, belladonna, or chloroform, or the ptomaines or toxins of
bacteria. Causes acting on the brain may, however, lead to death by
syncope or asphyxia when the nerve centres presiding over circulation or
respiration are the first to feel the full effects of the pressure or
poison.
=Death from old age=, with a gradual failure of the natural processes of
nutrition and tissue-growth, and the occurrence of atrophy and various
degenerations of the organs is not a common occurrence in domesticated
animals, so that it may be dismissed without further notice.
_Actual somatic death_ is marked by the cessation of breathing and
pulse, the dilated pupils and semi-closed eyelids, the coldness and
pallor of the visible mucous membranes and skin, and the clenching of
the jaws with slight protrusion of the tongue. Yet these symptoms may be
present in syncope and it may even be impossible to detect the beats of
the heart, though the subject still lives. Pressure of the finger on a
white portion of the skin or on a mucous membrane may give a further
indication. If the indentation made by the finger is slowly effaced and
if the blood again slightly reddens the part the presumption is against
death. Even this is not infallible, since by pressure of gas in the
internal cavities or deeper blood vessels, the blood may be forced back
into the surface capillaries giving an appearance of circulation, after
actual death. On the other hand any exudation or œdema will retain the
imprint of the finger even in life. The general relaxation of the
muscles and their lack of response to electric stimulation, and the
setting in of cadaveric rigidity, and later still of putrefaction give
more conclusive evidence of dissolution.